1. Field of the Invention
This disclosure is related to radiation measurements using scintillation type radiation detectors, and more specifically related to apparatus and methods for measuring radiation in a borehole environment using a YAlO3:Ce (YAP) scintillation crystal.
2. Background of the Art
Scintillation type radiation detectors have been used for decades in a wide variety of applications. Radiation absorbed by a scintillation crystal emits a pulse of light or “scintillates”. The intensity of light is a function of energy deposited within the crystal by the absorbed radiation. A measure of light intensity can, therefore, be related to the energy of radiation absorbed by the scintillator. A measure of the number of scintillations per unit time can be related to the intensity of radiation absorbed by the scintillation crystal.
In fabricating a scintillation type radiation detector, a scintillation crystal is optically coupled to a light sensitive device that responds to the number and to the intensity of scintillations produced within the crystal. Phomultiplier tubes (PMT) are commonly used as light sensitive devices. A PMT converts scintillations from the coupled crystal into electrical pulses. A pulse is typically generated for each scintillation. The magnitude of the pulse is proportional to the intensity of the scintillation. A count per unit time of pulses can, therefore, be related to the intensity of radiation impinging upon the crystal. Measures of magnitudes of the pulses can, therefore, be related to corresponding energies of the radiation absorbed by the crystal. Alternately, scintillation crystals can be optically coupled to other types of light sensitive devices such as photodiodes, and intensity and energy of impinging radiation can be determined from electrical outputs of these devices.
The scintillation process is not instantaneous and, in fact, the scintillation emission intensity follows an exponential decay. Thallium activated sodium iodide, or NaI(Tl), is a commonly used material in scintillation type gamma radiation detectors. The decay constant of a scintillation produced within a NaI(Tl) crystal by impinging gamma radiation is about 230 nanoseconds (ns). If the intensity of radiation impinging upon the crystal is sufficiently intense to generate a subsequent scintillation pulse before the previous scintillation pulse has decayed to a negligible level, the scintillation pulses will essentially “sum” within the crystal. This is commonly referred to as pulse “pile-up”. As an example, two pulses of equal intensity (induced by two gamma rays of equal energy) which pile-up within a detector system will produce a single electrical pulse output with a magnitude greater than a pulse that would be produced by a single gamma ray. Since pulse magnitude is related to radiation energy, pulse pile-up typically results in an erroneous radiation energy measurement. Furthermore, since the pulses “sum” as a single rather than a multiple radiation detector events, pulse pile-up results in erroneous radiation intensity measurements in high intensity gamma ray fluxes. It is, therefore, highly desirable to utilize a scintillation crystal with a minimum light decay constant when measuring energy and intensity of high intensity gamma radiation fluxes. As an example, there is a class of borehole instruments that employs a source of pulsed neutrons and one or more scintillation detectors. Certain measurements, such as inelastic scatter gamma ray measurements, require that the one or more detectors be operated during the neutron burst. This exposes the one or more detectors to extremely high fluxes of gamma ray and other types of radiation. The light decay constant of the scintillation material is, therefore, a critical design parameter in this type of instrumentation. Many measurement systems using scintillation type gamma ray detectors are also exposed to neutron fluxes. As in the example above, a large variety of borehole instruments used to measure properties of earth formation penetrated by the borehole employ one or more scintillation gamma ray detectors and a neutron source. The neutron source, whether pulsed or continuous, induces gamma radiation within the formation through several types of reactions including inelastic scatter and thermal capture. This induced gamma radiation is sensed by the one or more scintillation detectors and is used to determine formation and borehole parameters of interest. The scintillation detectors are also exposed to neutrons from the source, and especially to thermal neutrons generated in the borehole environs. These neutrons can produce radiation-emitting isotopes within the scintillation crystal. This is commonly referred to as crystal “activation”. Consider, as an example a scintillation detector comprising a NaI(Tl) crystal. The thermal neutron capture cross sections for the primary elemental constituents sodium (Na) and iodine (I) are 0.43 barns and 6.15 barns, respectively. Thermal neutrons impinging upon the NaI(Tl) detector produce 24Na and 128I within the scintillator through the 23Na(n,γ)24Na and 127I(n,γ)128I reactions, respectively. Both 24Na and 128I decay through beta emission with 128 I also decaying through electron capture. There is often gamma emission subsequent to the beta decay or electron capture. These radiations are generated within the NaI(Tl) crystal, and both the gamma and beta radiation induce scintillations within the crystal. These activation induced radiations are considered as “noise” in the measurement of formation properties using gamma radiation induced within the formation and borehole. It is, therefore, highly desirable to use a scintillation crystal with primary elemental constituents that do not readily “activate” when used in a system which also utilizes a neutron source.
There are other considerations in selecting a scintillation crystal for borehole applications. The borehole environment is typically harsh in that pressures and temperatures are typically high. Borehole instruments are subjected to shock and vibrations as the instrument is typically conveyed within the borehole. Crystals such as NaI(Tl) are highly susceptible to shock induced cleavage, which typically worsens with constant vibration. Cleavage, in turn, results in deteriorating energy resolution and efficiency. As mentioned previously, temperature is usually elevated within a borehole, and typically varies with depth. In particular, variations in temperature can adversely affect crystal scintillation properties of a crystal which, in turn, can adversely affect subsequent radiation energy and intensity measurements. Some scintillation crystals, such as NaI(Tl), are hygroscopic. This requires that the crystal be encased in a hermetically sealed container, which increases the overall dimensions of the crystal package for a given active crystal volume. This increase in size, or the resulting necessity to reduce the active volume of the crystal, can be a critical design factor in borehole instrument fabrication. Inherent crystal gamma ray resolution properties and overall efficiency properties are also factors in borehole logging instrument design.
Other scintillation crystals have been used in borehole applications. Typically, these scintillation materials exhibit advantages over NaI(Tl) in some areas, but exhibit disadvantages in other areas. On such material is bismuth germinate (BGO), with properties well documented in the literature.
The scintillation material cerium activated yttrium aluminum perovskite or YAlO3:Ce (YAP) has a density of 5.55 grams per cubic centimeter (g/cm3), an effective Z of 36, a light decay constant of 27 ns, light output of 45% of NaI at 25° C., 18,000 photons/MeV, emission peak of 350 nanometer (nm), and an index of refraction of 1.94. Thermal neutron cross sections for the major constituents of the crystal yttrium, aluminum and oxygen are 1.28 barns, 0.230 barns and 0.00019 barns, respectively. The activity produced by thermal neutron capture in yttruim is relatively long-lived so that decay radiation is negligible compared to that observed from iodine activation in NaI crystals.
YAP has been used in the prior art in a number of non-borehole scintillation detector applications, and especially in the field of medical imaging. Typical prior art applications are summarized below.
A gamma ray camera system comprising an array of YAP(Ce) scintillation crystals optically coupled to a position sensitive photomultiplier tube is disclosed in “YAP Multi-Crystal Gamma Camera Prototype”, K. Blazek et al, IEEE Transactions on Nuclear Science, Vol. 42, No. 5, October 1995. The multiple scintillation crystals are optically isolated from one another.
A scintillator detector with multiple YAP crystals and other types of crystals is disclosed in “Blue Enhanced Large Area Avalanche Photodiodes in Scintillation Detection with LSO, YAP and LuAP Crystals”, M. Moszynski et al, IEEE Transactions on Nuclear Science, Vol. 44, No. 3, June 1997. Scintillator crystals are optically coupled to large area avalanche photodiodes.
A high resolution positron emission tomograph (TierPET) for imaging small laboratory animals is discloses in “Recent Results of the TierPET Scanner”, S. Weber et al, IEEE Transactions on Nuclear Science, Vol. 47, No. 4, August 2000. The system is based on an array of YAP crystals. 20×20 arrays of 2×2×15 mm polished YAP crystals are optically coupled to a position sensitive PMT.
U.S. Pat. No. 5,313,504 to John B. Czirr discloses the use of a YAP scintillator in a borehole instrument to monitor output of a neutron source that is also disposed within the borehole instrument. Since the YAP scintillator is used in a neutron source monitor system, the instrument is designed to maximize the response of the YAP scintillator to the neutron source and, conversely, to minimize the response of the YAP scintillator to the borehole environs.
None of the above cited references discloses a system that is suitable for operation within a borehole to measure properties of the borehole environs.